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Fig. 3. Preparation of C-terminal CF 3 -isomerization.Pro containing dipeptides and cis/transCoupling reactions at the deactivated N-terminal position were first performed following areported procedure [9] and required the use of amino acid halides. Optimization of thereaction allowed the Fmoc-peptide synthesis in base free condition in excellent yield withglycine and alanine (Figure 3). A moderate yield was obtained for the bulky Aib aminoacid. In all cases, starting from (2R,5R)- or (2R,5S)-oxazolidine, an epimerization occurredat C 2 center to give the major (2R,5S)-oxazolidine containing dipeptide diastereomer A.This result was confirmed by nOe experiments showing the presence of nOe signal betweenH and H . Conformational study of the cis/trans isomerization ratio of the amide bond forA by nOe indicates that the cis conformation is largely favored from the moment that R 1group is not H.AcknowledgmentsWe thank the CNRS for the grant to O.B. and Central Glass Co. for financial support and the gift oftrifluoroacetaldehyde hemiacetal. We also thank the French Fluorine Network.References1. Dumy, P., Keller, M., Ryan, D.E., Rohwedder, B., Wohr, T., Mutter, M. J. Am. Chem. Soc. 119,918-925 (1997).2. Keller, M., Sager, C., Dumy, P., Schutkowski, M.; Fisher, G.S., Mutter, M. J. Am. Chem. Soc. 120,2714-2720 (1998).3. Huguenot, F., Brigaud, T. J. Org. Chem. 71, 7075-7078 (2006).4. Chaume, G., Van Severen, M.C., Marinkovic, S., Brigaud, T. Org. Lett. 8, 6123-6126 (2006).5. Chaume, G., Van Severen, M.C., Ricard, L., Brigaud, T. J. Fluorine Chem. 129, 1104-1109 (2008).6. Caupène, C., Chaume, G., Ricard, L., Brigaud, T. Org. Lett. 11, 209-212 (2009).7. Chaume, G., Lensen, N., Caupène, C., Brigaud, T. Eur. J. Org. Chem. 5717-5724 (2009).8. Chaume, G., Barbeau, O., Lesot, P., Brigaud, T. J. Org. Chem. 75, 4135-4145 (2010).9. DalPozzo, A., Bergonzi, R., Ni, M. Tetrahedron Lett. 42, 3925-3927 (2001).77